1. Field of the Invention
This invention relates generally to a method using directed self-assembly of block copolymers for making an imprint template for imprinting patterned-media magnetic recording disks.
2. Description of the Related Art
Magnetic recording hard disk drives with patterned magnetic recording media have been proposed to increase data density. In patterned media, also called bit-patterned media (BPM), the magnetic recording layer on the disk is patterned into small isolated data islands separated by nonmagnetic spaces or trenches and arranged in concentric generally circular data tracks. To produce the required magnetic isolation of the patterned data islands, the magnetic moment of spaces between the islands must be destroyed or substantially reduced to render these spaces essentially nonmagnetic. In one type of BPM the magnetic material is deposited on a flat disk substrate. The magnetic data islands are then formed by milling, etching or ion-bombarding of the area surrounding the data islands. BPM disks may be perpendicular magnetic recording disks, wherein the magnetization directions are perpendicular to or out-of-the-plane of the recording layer.
One proposed method for fabricating BPM disks is by imprinting with a template, sometimes also called a “stamper”, that has a topographic surface pattern. In this method the magnetic recording disk substrate with a polymer film on its surface is pressed against the template. The patterns on the template may be protrusions (pillars), or recesses (holes), and the type of the pattern is referred to as the polarity of the template. The polymer film receives the reverse image of the template pattern and then, depending on the polarity of the template, either becomes a mask for subsequent etching of the disk substrate if the template is hole-type, or becomes a sacrificial layer for a tone reversal process such as liftoff followed by etching of the disk substrate, if the template is pillar type, to form pillars on the disk in both cases. In one type of BPM disk, the magnetic layer and other layers needed for the magnetic recording disk are then deposited onto the etched disk substrate and the tops of the pillars to form the BPM disk. In another type of BPM, the magnetic layers and other layers needed for the magnetic recording disk are first deposited on the flat disk substrate. The polymer film used with imprinting is then pressed on top of these layers. The polymer film receives the reverse image of the template pattern and then becomes a mask, or sacrificial layer for tone reversal, for subsequent milling, etching or ion-bombarding the underlying layers.
The template may be a master template for directly imprinting the disks. However, the more likely approach is to fabricate a master template with a pattern of pillars corresponding to the pattern of pillars desired for the disks and to use this master template to fabricate replica templates using imprint lithography. The replica templates may have the opposite or same polarity of the master template. The replica templates are then used to directly imprint the disks.
The making of a master imprint template is a difficult and challenging process. To achieve BPM disks with an ultra-high areal bit density greater than 1 Terabits/in2, a track pitch and an island pitch of about 20 nm will be required. Directed self-assembly of block copolymers (BCPs) has been proposed for making the template and is believed capable of achieving areal bit densities of greater than 1 Terabit/in2. U.S. Pat. No. 7,976,715 B2, assigned to the same assignee as this application, describes the use of directed or guided self-assembly of block copolymers to form a pattern of generally radial lines on a template substrate, followed by conventional lithography to form a pattern of concentric generally circular rings over the radial lines. After etching of the substrate and removal of resist, the substrate has a pattern of protrusions of the other block copolymer component, which are then used as an etch mask to etch the substrate into a pattern for imprinting disks with discrete data islands arranged in concentric generally circular data tracks.
Another method to generate a master template containing a pattern of pillars corresponding to the pattern of data islands is to combine the patterns from two submaster templates using separate imprint steps. Each imprint submaster template is created using directed self-assembly of BCPs, and may also require line-doubling techniques for even smaller pattern size and higher density. Pending application Ser. No. 13/627,492, filed Sep. 26, 2012 and assigned to the same assignee as this application, describes the use of two such imprint submaster templates, one with a pattern of generally radial spokes or lines, and the other with generally concentric rings, to make the master template by two separate imprinting steps with the two submaster templates.
Like conventional non-patterned disks, BPM disks are also required to have non-data servo regions that are used for read/write head positioning. However, unlike the data islands, which are generally the same size and shape and are arranged in a periodic pattern, the servo islands typically have different sizes and shapes and do not form a periodic pattern. The servo trenches are typically much wider than the data trenches. The servo islands are thus not capable of being formed by self-assembly of BCPs so the servo regions must be masked during formation of the periodic patterns in the data regions and then the servo islands formed by separate lithographic processing steps.
What is needed is a method for making an imprint template using self-assembly of BCPs that can result in the simultaneous formation of the desired high-density patterns in the data regions and the required servo islands in the servo regions.